1. Field
The present invention relates to a voltage converting devices and a voltage converting method.
2. Description of the Related Art
In general, when a voltage is supplied from a power supply, such as a direct-current (DC) power supply or an alternating-current (AC) power supply, to a load circuit, a switching converter may be used so that the voltage required for operating the load circuit is stably supplied. The switching converter includes switching elements for quick switching, such as transistors, controlling a duty ratio and switching frequency of the switching elements to convert a power-supply voltage input from the power supply to a desired output voltage.
FIG. 14 is a drawing of an example of configuration of a general DC/DC converter that converts a power-supply voltage Vin of a DC power supply 11 to an output voltage Vo to be applied to a load circuit 16. The DC/DC converter depicted in the drawing includes a transistor 12, a transistor 13, a coil 14, a capacitor 15, an operational amplifier 17, and a Pulse Width Modulation (PWM) circuit 18. The DC/DC converter depicted in FIG. 14 is a switching converter that always outputs the constant output voltage Vo even when an output current io flowing into the load circuit 16 changes due to the operational state.
The transistors 12 and 13 perform operations reverse to each other, with one being in a continuity state while the other being in an interruption state. Furthermore, the longer a period over which the transistor 12 is in a continuity state, the more the output voltage Vo from the DC/DC converter. Switching between a continuity state and an interruption state in the transistors 12 and 13 is controlled with a pulse output from the PWM circuit 18. That is, the transistor 12 is in a continuity state during a period over which the pulse output from the PWM circuit 18 is in an ON state, and the transistor 13 is in a continuity state during a period over which the pulse is in an OFF state.
When a desired rectangular voltage is obtained through switching of the transistors 12 and 13, the voltage is rectified and smoothed by the coil 14 and the capacitor 15, and then the output voltage Vo is output from the DC/DC converter. The output voltage Vois also input to the operational amplifier 17 provided to a feedback loop. In the operational amplifier 17, the output voltage Vo and a reference voltage are differential-amplified, and a difference is then fed back to the PWM circuit 18.
The PWM circuit 18 adjusts a pulse width for switching between a continuity state and an interruption state of the transistors 12 and 13 according to the feedback from the operational amplifier 17, thereby controlling the continuity state and interruption state of the transistors 12 and 13. Specifically, when the output voltage Vo is greater than the reference voltage and the difference output from the operational amplifier 17 is positive, the PWM circuit 18 decreases a duty ratio to shorten the period over which the transistor 12 is in a continuity state. On the other hand, if the output voltage Vo is equal to or smaller than the reference voltage and the difference output from the operational amplifier 17 is negative, the PWM circuit 18 increases a duty ratio to extend the period over which the transistor 12 is in a continuity state.
With such feedback control, the duty ratio is adjusted so that the output voltage Vo and the reference voltage always coincide with each other. As a result, even if the output current io flowing through the load circuit 16 changes, the output voltage Vo is always kept constant, thereby ensuring a voltage required for the load circuit 16 to operate. Such a conventional converter is exemplarily disclosed in Japanese Patent Application Laid-open No. 2000-227808.
However, in the load circuit to which the output voltage from the switching converter is supplied, when the current abruptly changes, feedback control in the switching converter cannot sufficiently follow this change, disadvantageously inviting a temporary decrease or increase of the output voltage. Specifically, for example, when the output current io flowing through the load circuit 16 depicted in FIG. 14 abruptly changes, feedback control by the operational amplifier 17 and the PWM circuit 18 falls behind. With this, the output voltage Vo may temporarily exceed an allowable range of the load circuit 16.
In particular, in recent years, a standby current flowing at the time of idling of the load circuit 16 tends to be reduced as much as possible in view of consideration to the terrestrial environment and others. For this reason, at the time of changing from an idling state to an operational state, the output current io flowing through the load circuit 16 abruptly increases at a speed on the order of, for example, ten to several hundreds A/μS (amperes/microseconds). As a result, a delay in response of the operational amplifier 17 and the PWM circuit 18 depicted in FIG. 14 occurs, thereby temporarily decreasing the output voltage Vo.
That is, for example, as depicted in FIG. 15, when the output current io abruptly changes, the output voltage Vo decreases from a voltage Vc to a voltage Vc′. At this time, when the voltage Vc′ falls short of an operating voltage of the load circuit 16, the load circuit 16 may fail to work properly or stop operation. Conversely, when the output current io abruptly decreases, the output voltage Vo temporarily increases, thereby causing an excessively-large voltage to be applied to the load circuit 16, leading to the occurrence of, for example, oscillation of an amplifier in the load circuit 16.
Normally, as for a change of the output current io, the capacitor 15 depicted in FIG. 14 has a function of supplying a current, thereby suppressing an increase or decrease of the output voltage Vo with a charge stored in the capacitor 15 when the output current io slightly changes. However, when the change amount of the output current io is large, some measures have to be taken, such as increasing the capacity of the capacitor 15 or disposing many capacitors 15. Thus, if an abrupt and relatively-large change of the output current io is taken into consideration, the circuitry size in the switching converter and cost thereof have to be disadvantageously increased.